This invention relates to a hydraulic tensioner having a check valve assembly which is actuated by the position of a piston.
Hydraulic tensioners are typically used as a control device for a chain drive system in an automobile timing system. The tension in the chain can vary greatly due to the wide variation in the temperature and the linear thermal expansion among the various parts of the engine. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain. A hydraulic tensioner is used to take up the slack in the chain or belt that connects the camshafts to the crankshaft of the engine timing system.
A typical hydraulic tensioner is comprised of a housing having a bore, a fluid chamber defined by the bore, and a hollow piston biased in a protruding direction from the bore by a spring. A check valve is also included in the hydraulic tensioner to permit fluid flow from a source of pressurized fluid into the fluid chamber, while preventing back flow in the reverse direction. The force of the chain against the piston in an inward direction is balanced by the resistance force of the fluid and force of the spring in an outward direction.
The check valve assembly is optimally designed to permit fast, instantaneous flow into the chamber and limited or no flow in the reverse direction. The response of the valve to changes in flow is constrained by the mass of the ball or disc, the size of the flow orifice, and the preload and spring rates of the check valve spring. During start-up, the piston spring forces the piston outward to create a vacuum condition in the chamber, which causes the check valve to open and permit flow into the chamber. Until the oil temperature rises to the high temperature level of normal operating conditions, the viscosity of the oil is high and flow through the check valve is slow. In such a condition, the oil cannot enter the chamber fast enough to match the expansion of the chamber from the rising piston. Thus, the chamber does not contain enough oil to properly damp the chain. This condition of low pressure in the chamber without sufficient fluid is known as cavitation and can be damaging to both the tensioner and chain.
In Kawashima et al., U.S. Pat. No. 4,940,447, the problem of cavitation is addressed by using a retainer mounted under the piston with a flat check plate. The plate is movable between two positions to open and close the channel or flow conduit into the chamber. The channel in the piston can be opened wide with a minimum stroke of the check plate to allow a large amount of oil to flow through the channel into the pressure chamber. Thus, cavitation is minimized by the quick response of the flat check plate valve.
In the present invention, in various embodiments, a check valve assembly is used to address the problems of: (i) cavitation associated with a ball check valve, (ii) tipping associated with standard flat disc type check valves, and (iii) concentricity associated with a contoured disc type check valve. A check valve assembly having a contoured disc as set forth in one embodiment of the invention provides a line type contact with the check valve seat, rather than a full surface type contact that would normally occur with a flat disc. The line contact and associated curvature allow for a less restrictive oil flow path between the disc and the seat.
Generally the performance of a check valve is constrained by the mass of the valve's moving components, the size of the flow orifice as the valve opens, and the spring preload and spring rate. The inlet orifice size of the valve of the present invention is large enough to allow a high flowrate of low temperature oil into the chamber so as to minimize the possibility of cavitation in the chamber.